A hybrid engine is one in which more than one prime mover contributes power to a single power output. Hybrid engine technology is currently a field of active research and development in the effort to improve the fuel efficiency of heat engines where a heat engine is a device capable of converting heat into mechanical work. An internal combustion engine is a typical heat engine.
The ratio between the energy input to the engine, measured as the calorific value of the fuel multiplied by the rate of fuel flow, and the work output from the engine is called the thermal efficiency. The thermal efficiency of a reciprocating internal combustion engine, such as an automotive engine, may be of the order of 30-40%. Thus 60-70% of the energy contained in the fuel is wasted. This wastage may partly be heat rejected by the engine, which is typically inherent in the functioning of heat engines, partly mechanical friction inside the engine and partly noise emitted by the engine.
Heat rejected may typically appear as:                (a) Heating of the cylinder walls and other engine parts. This heat may be dissipated to atmosphere through air or liquid cooling systems in order to prevent damage to the cylinder(s) and other engine parts.        (b) Hot exhaust gases. Exhaust gas heat may be dissipated to atmosphere through the walls of the exhaust manifold, muffler and exhaust pipe and as a final discharge of warm gases.        
Waste heat from internal combustion engines may sometimes be used for heating the interiors of buildings and vehicles but, especially in the case of automotive engines, the proportion of the total heat wastage used for this purpose may typically be very small.
Exhaust gases may typically leave the cylinders of a reciprocating internal combustion engine at temperatures of the order of 1,000° F. Their final exit temperature from the exhaust pipe may be of the order of 100° F.
In a reciprocating steam engine, steam may typically enter the cylinders at temperatures of the order of 500-700° F. and leave the engine at temperatures of the order of 250° F. The temperature range in which a steam engine functions may therefore lie within the range between the initial and final exhaust gas temperatures of a typical internal combustion engine.
A hybrid engine could therefore comprise a primary internal combustion engine with a secondary steam engine using the primary engine's waste heat and adding to the hybrid engine's power output and thermal efficiency. The following references disclose means of generating and using steam from an internal combustion engine exhaust gas.
U.S. Pat. No. 4,300,353 to Ridgway teaches a hybrid engine of the type described above in which the steam is generated in a boiler.
U.S. Pat. No. 4,433,548 to Hallstrom teaches a hybrid internal combustion/steam engine and specifies that steam is generated in a generating chamber by the transfer of heat to water from the hot surfaces of the chamber.
U.S. Pat. No. 4,406,127 to Dunn teaches a hybrid internal combustion/steam engine in which water is sprayed onto the hot surface of an exhaust manifold inside a steam-generating chamber. The resulting steam is used in a closed-circuit reciprocating steam engine.
U.S. Pat. No. 5,000,003 to Wicks discloses a hybrid internal combustion/steam engine in which steam is generated in a boiler and used in a closed-circuit steam engine.
U.S. Pat. No. 5,010,852 to Milisavlevic teaches a multi-fuel, multi-hybrid engine in which part of the power output is provided by steam generated in a boiler.
U.S. Pat. No. 5,191,766 to Vines discloses a hybrid internal combustion/steam engine in which steam is generated in a steam generation chamber which is separated by valving from both the internal combustion cylinder and the means of using the steam. Specifically, the steam is generated in a generation chamber, stored in a compression tank and released to drive a steam turbine operating in closed-circuit with a condenser. The steam generation system is interposed between the internal combustion engine cylinder and the means of using the steam. The transfer of water and steam between the components of the system is handled by valving.
U.S. Pat. No. 6,202,782 to Hatanaka discloses a hybrid engine in which heat is stored and periodically released in a closed-circuit gas turbine system.
U.S. Pat. No. 7,047,722 to Filippone teaches a hybrid internal combustion/steam engine in which the steam is generated and used in a closed-circuit turbine.
In all patents except U.S. Pat. No. 5,191,766, the steam and exhaust gases are separated from each other by heat-transfer walls.
A significant problem with the hybrid internal combustion/steam engines disclosed and under development to date lies in their complexity, bulk, weight and potentially high construction and maintenance costs per unit of power output.
FIG. 1 shows a block diagram of a typical internal combustion/steam hybrid engine 2. The steam engine in this example is a closed-circuit, turbine-type steam engine.
Considering FIG. 1, typically, the quasi-continuous flow of exhaust gases from the internal combustion engine 4 may be used to heat water in a boiler 6. Steam is generated under pressure in the boiler, may be further heated in a superheater 8 and may then be expanded in a turbine-type steam engine 10. The steam may then be passed through a condenser 12. The condensate water may then be pumped back into the boiler 6 by a feed pump 14 powered by the internal combustion engine.
The rotational speed and speed-torque characteristics of the turbine 10 may typically differ from that of the internal combustion engine 4. Consequently, the turbine may drive an electric generator 16 which drives an electric motor 18, the power output of which may then be applied to the hybrid engine drive shaft.
Whether the steam circuit is closed (with a condenser 12 returning exhaust steam to the boiler 6 as water) or open (exhausting steam to atmosphere), the boiler 6 requires an injector or feed pump (not shown) to force water into the boiler against the pressure of the steam being generated there. The injector or feed pump consumes some of the power produced by the hybrid engine.
While these arrangements may tend to maximize thermal efficiency, they may also tend to make a hybrid internal combustion/steam engine substantially bulkier, heavier and more complicated than a conventional internal combustion engine of equivalent power output and, hence, more costly to construct and maintain, thereby detracting from its total economy.